1. Field of the Invention
The present invention relates to an integrated sprocket and housing which is, in particular, used in a variable valve timing mechanism, and which includes a sprocket portion which is formed in a substantially annular shape, and which has teeth on the outer circumference thereof, and a housing portion which is disposed inside the sprocket portion, and which has recesses in the inside thereof. The present invention also relates to a method for manufacturing an integrated sprocket and housing.
Priority is claimed on Japanese Patent application No. 2002-275411, filed Sep. 20, 2002, which is incorporated herein by reference.
2. Description of Related Art
In internal combustion engines installed in automobiles, variable valve timing mechanisms, by which open and close timing (valve timing) is changed, have been employed, in order to improve the efficiency of combustion in a low revolution range as well as in a high revolution range, and also to decrease exhaust gas.
A type of variable valve timing mechanism is known in the art, which includes a first rotational body (an inner rotor) which is connected to a camshaft so as to rotate, and a second rotational body (a housing) which is disposed coaxially with the first rotational body, and which is connected to a crankshaft so as to rotate with a sprocket (a driven gear), wherein a rotational phase is changed by rotating the first and second rotational body with respect to each other so that the valve timing is changed (see, for example, Japanese Unexamined Patent Application, First Publication No. Hei 11-93628).
In this case, in order to rotate the first and second rotational bodies (i.e., the inner rotor and housing) with respect to each other, pressure chambers are formed inside the housing, each of which is delimited by two vanes projecting outwardly from the outer circumference of the inner rotor and an inner circumferential wall of the housing, and a pressure difference is generated between two pressure chambers so that the vane disposed between the two pressure chambers is moved while sliding along the inner circumferential wall of the housing. As a result, the rotational phase between the camshaft and the crankshaft is changed so that the valve timing is changed.
In such variable valve timing mechanisms, the sprocket, which is driven by a chain, must have high surface pressure resistance, high tenacity, and high hardness in addition to low friction performance. On the other hand, the housing, on which the vane slides, must have high accuracy in shape, excellent wear resistance, and low friction performance.
The sprocket and housing rotate together; however, their requirements, such as above mechanical properties, are different; therefore, conventionally, the sprocket and housing are separately made from different materials, and made by applying different surface treatments, and then are assembled together.
A vane for a rotary compressor, an element which must have excellent wear resistance, is disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-342981. The vane is manufactured by powder-forming and sintering a ferrous powder material having sufficient hardenability, and through various subsequent treatments.
After increasing the strength through quenching and annealing after sintering, the vane is subjected to a steam treatment in order to improve the sealing performance, and is further subjected to a nitriding treatment (a gas soft nitriding treatment) in order to improve wear resistance. After the steam treatment and nitriding treatment, surface finishing by grinding is applied to improve the surface roughness and accuracy in shape.
In the field of variable valve timing mechanisms, reductions in manufacturing time and cost by reducing assembling steps are required, and it is desired to integrally manufacture the housing and sprocket by powder forming and sintering.
However, a problem is encountered in that it is difficult to manufacture a housing, which must be manufactured with a precise shape, by applying various treatments to a sintered compact that is made conventionally because dimensional control is difficult.
As mentioned above, the housing, which has a slide surface for the vane, must have low friction performance, excellent wear resistance, and high accuracy in shape. On the other hand, the sprocket, which is driven by a chain, must also have high strength. When the sprocket and housing, which are conventionally made separately through respective preferred processes, are integrally manufactured, the requirements such as strength, accuracy, and low friction cannot be satisfied at the same time because different requirements are desired for different portions.